The present invention relates to a method for improving step metal coverage of a semiconductor device, and more particularly to methods of making a semiconductor device using enhanced reflow of phosphosilicate glass by ion implantation of arsenic. The enhanced reflow smooths and rounds step profiles in the phosphosilicate glass, such as a contact window to the substrate through the phosphosilicate glass. The smoothed and rounded profiles result in producing better step metal coverage.
In the fabrication of semiconductor devices and integrated circuits one of the major concerns is the ability of metal lines to cross over abrupt steps and to enter contacts without breaking or thinning. Breakage will cause open circuit failures while thinning will cause a reliability hazard. And if metal lines are fabricated by sputtering or evaporation techniques over abrupt steps, steps due either to an underlying layer having a step profile or to abrupt edges of a layer, such breakage and thinning are almost inherent. In sputtering and evaporation techniques, the abrupt steps cause shadowing of the metal layer being fabricated; the shadowing in turn leads to metal thinning or cracking, or both, in the regions of these steps.
To overcome this problem, one popular approach has been to use phosphorous-doped, low temperature oxide of phosphosilicate glass (PSG) as the dielectric between metal and the substrate, for example, polycrystalline silicon. By flowing the PSG at sufficiently high temperature, which is typically 1000-1100 degrees C., sharp profiles are rounded out, thereby producing better step metal coverage. However, in next generation very large scale integrated circuit (VLSI) processing, such high temperatures will be intolerable due to dopant diffusion and must be reduced. The flow temperature can be reduced using steam ambients, but this leads collaterally to an undesirable oxidation of underlying silicon and to higher island sheet resistance. And if contacts have been etched prior to flow, then steam ambients should definitely be avoided. Otherwise, undesired oxidation occurs.
U.S. Pat. No. 4,319,260 and U.S. Pat. No. 4,355,454 disclose another method of reduced temperature by in situ doping of oxide, viz., using arsine to dope the glass. However, arsine is highly toxic and has not been widely used by the industry.
Borophosphosilicate glass has also been used to reduce temperature. Though giving enhanced flow it presents other problems, such as surface leaching of boron and recrystallization of boric acid crystals. These problems have tended to discourage the wider use of this technique.